Conventionally, as disclosed in, for example, JP S57-44799A, the strength of such a radiator fan can be maintained while restricting its length in the axial direction, and it can achieve efficient air flow.
However, if an engine is accommodated in an engine room and its radiator is cooled by a radiator fan, then, as shown in FIG. 11, the state of the engine cooling air flow is determined by the intersection point (1) of the conventional fan characteristics (shown as a thin broken line in FIG. 11) and the airflow resistance within a conventional engine room (shown as thick broken line in FIG. 11). And, under these conditions (intersection point (1) in FIG. 11), a relative noise of the radiator fan is determined by the characteristics of the conventional fan, as shown in FIG. 10. In this case, the relative noise (in dB) given on the vertical axis of FIG. 10 is a normalized value of the measured fan noise SL, and is a value that can be determined by SL−10×log(0.624×P2×Q), where P(Pa) is the static pressure in the airflow from the radiator fan and Q(m3/s) is the flow amount, allowing comparison of equivalent flow conditions (static pressure, flow) when comparing fan noise.
Further, the pressure coefficient (dimensionless) of the vertical axis of FIG. 11 is a nondimensionalized value of the static pressure, and can be determined by P/(0.5×π×ρ×(H×Df)2), where ρ(kg/m3) is air density, H (1/s) is a fan rotational frequency and Df is a fan diameter. The flow coefficient (dimensionless) on the horizontal axis of FIG. 10 and FIG. 11 is a nondimensionalized value of the flow, and can be calculated by Q/(0.25×π2×H×Df3). In all following diagrams, the definitions of the relative noise, the pressure coefficient and the flow coefficient are the same, so that they will not be explained further.
In this type of situation, if the airtightness of the engine room is increased to prevent leakage of engine noise to the outside, then, as shown in FIG. 11, there will be a change to the airflow resistance within the engine room, and the intersection point with the characteristic curve of the conventional fan will move from point (1) to point (2). Accordingly, for the fan with the conventional characteristics as given in FIG. 10, although the engine noise is less likely to leak out, relative noise will be increased instead, and with respect to noise on the outside of the engine room, the radiator fan becomes a new source of noise.
Thus, a problem to be solved by the present invention is to provide a radiator fan which can suppress noise generation even when used within an engine room of high airtightness, and an engine cooling device using such a fan.